Recently, water absorbent resins have been used extensively as a component of sanitary articles, such as paper diapers, sanitary napkins, and incontinence pads, to absorb body fluids. Examples of such water absorbent resins include: a partially neutralized crosslinked polymer of polyacrylic acid; a hydrolyzed graft polymer of starch and acrylic acid; a saponified copolymer of vinyl acetate and acrylic ester; a hydrolyzed copolymer of acrylonitrile or acrylamide, or a crosslinked product thereof; and a crosslinked polymer of cationic monomers.
Such a water absorbent resin has been required to have excellent properties, including an ability to absorb a sufficient amount of liquid at a sufficient speed when in contact with an aqueous fluid like a body fluid. Other required properties include gel strength, gel permeability, and absorbing power for absorbing liquid from a base material containing an aqueous fluid. Further, in recent years, there has been an ongoing demand for a water absorbent resin powder having a very narrow particle size distribution, or that with high absorbency and low water-soluble content. It is also necessary that such water absorbent resin powder has high absorbency against pressure, and liquid permeability against pressure.
For example, Documents 1 through 24 below disclose parameters and measurement methods for specifying properties of such a water absorbent resin.
Specifically, Document 1 (U.S. Reissue Pat. No. 32,649) proposes a water absorbent resin that excels in gel strength, soluble content, and absorbency. Document 2 (U.K. Patent No. 2,267,094B) proposes a water absorbent resin with excellent liquid permeability against no pressure, and excellent absorption speed and absorbency. Further, Document 3 (U.S. Pat. No. 5,061,259), Document 4 (U.S. Pat. No. 5,419,956), Document 5 (U.S. Pat. No. 6,087,002), and Document 6 (European Patent No. 629,411) propose techniques for specifying a particle size distribution.
Further, many types of water absorbent resins with excellent absorbency against varying pressures and their measurement methods have been proposed. For example, there has been proposed water absorbent resins whose properties include absorbency against pressure either by itself or in combination with other properties, as disclosed in Document 7 (European Patent No. 707,603), Document 8 (European Patent No. 712,659), Document 9 (European Patent No. 1,029,886), Document 10 (U.S. Pat. No. 5,462,972), Document 11 (U.S. Pat. No. 5,453,323), Document 12 (U.S. Pat. No. 5,797,893), Document 13 (U.S. Pat. No. 6,127,454), Document 14 (U.S. Pat. No. 6,184,433), Document 15 (U.S. Pat. No. 6,297,335), and Document 16 (U.S. Reissue Pat. No. Re37,021).
Further, water absorbent resins with excellent impact resistance in terms of property degradation have been documented, as disclosed in Document 17 (U.S. Pat. No. 5,140,076) and Document 18 (U.S. Pat. No. 6,414,214B1). Further, a water absorbent resin that specifies an amount of dust is proposed in, for example, Document 19 (U.S. Pat. No. 5,994,440), and a water absorbent resin with little coloring is proposed, for example, in Document 20 (U.S. Pat. No. 6,444,744). As to resistance to urine, Document 21 (U.S. Pat. No. 6,194,531) and Document 22 (European Patent No. 940,148) propose water absorbent resins with excellent gel durability and absorptive power with respect to a liquid such as an aqueous solution of L-ascorbic acid, and Document 23 (European Patent No. 1,153,656) discloses a water absorbent resin with excellent breathability. Further, a water absorbent resin with less residual monomer is proposed in Document 24 (European Patent No. 605,215).
It is also known that water absorbent resins with specific properties are suitable for absorbent articles (diapers) having particular properties, configuration, and/or polymer concentration, as disclosed in Document 25 (U.S. Pat. No. 5,147,343), Document 26 (U.S. Pat. No. 5,149,335), Document 27 (European Patent No. 532,002), Document 28 (U.S. Pat. No. 5,601,542), Document 29 (U.S. Pat. No. 5,562,646), Document 30 (U.S. Pat. No. 5,669,894), Document 31 (U.S. Pat. No. 6,150,582), Document 32 (International Publication WO02/53198), Document 33 (U.S. Pat. No. 5,843,059), and Document 34 (U.S. Patent Application No. 2001/49514).
The water absorbent resins have thus been developed in view of these different properties, and some of them have been manufactured and used by targeting and specifying these properties. However, a problem remains that, even with the control of specific properties (performance), the water absorbent resins still cannot deliver sufficient performance in practical use as in paper diaper applications. Particularly, the performance of the water absorbent resins is not sufficient when they are used in sanitary products that contain an increased amount of water absorbent resin and a decreased amount of fiber material (high concentration of water absorbent resin).